1. Field of the Invention
This invention relates to position measuring transducers and more particularly to a position measuring transducer having two relatively movable members bearing electrostatically coupled electrodes.
2. Description of the Prior Art
Precision position measuring transducers of the resolver type using the effect of magnetic coupling between multipole transformer windings on relatively movable members are well known in the art. In order to increase the precision of this type of transducer, however, it is necessary to reduce the spacing between transformer turns, thereby reducing the cross-sectional area of the current carrying windings (which are typically in the form of thin printed copper layers). As this area is reduced, the I.sup.2 R loss in the windings is increased, and the effective gain of the transducer is drastically cut, effectively setting a limit on practicably attainable precision of this type of transducer.
In order to overcome this disadvantage, the present disclosure teaches a novel, high gain position measuring transducer which does not employ magnetic inductive coupling effects, but instead uses electrostatic effects between electrodes arrayed on relatively movable members. Since these electrodes are not required to carry appreciable currents, their dimensions can be miniaturized and transducer precision increased without an accompanying serious decrease in transducer gain.
In addition to the increased precision inherent in the miniaturization of the electrode dimensions, such miniaturization permits the use of ultra-thin electrodes which can be produced by photoetching techniques with much greater positional accuracy than can thicker electrodes. Ultra-thin electrodes of very narrow width also have greater mechanical adherence and stability than thicker electrodes of the same width because they are less subject to undercutting by the etching process which tends to reduce the area of the interface between the electrodes and the surface on which they are deposited.
Position measuring devices using electrostatic effects to generate electrical signals in reponse to angular or linear displacement of two relatively movable members have previously been proposed. One such device, disclosed in U.S. Pat. No. 2,674,729 to Carter, has two relatively rotatable disks separated by a narrow air gap, each disk bearing a large number of opposed interleaved finger-like electrodes. Excitation of the electrodes on one disk induces a voltage across the opposing electrodes on the other disk, and this voltage varies as one disk is rotated with respect to the other, giving an indication of their relative angular displacement.
Another electrostatic position measuring device, shown in U.S. Pat. No. 3,125,716 to Machlis, comprises a phase shifting apparatus which employs two sets of conjugate sinusoidal conductive patterns mounted on a stator element together with two output conductors. Relatively movable coupling elements are arrayed on a rotor element in spaced opposition to the sinusoidal patterns, which are excited by an alternating voltage source. An output voltage is electrostatically induced in the output conductors, the phase of the output voltage varying as a function of the relative angular displacement of the rotor and stator elements.
However, serious difficulties have been encountered with these electrostatic devices of the prior art. For example, spurious electrostatic coupling between input and output electrodes has required the use of specialized shielding circuitry such as that described in U.S. Pat. No. 3,219,920 to wall. Undesired harmonics are present in the output signal of electrostatic devices of the prior art. This impairs the accuracy of such devices when employed for position measuring purposes. Additionally, when the angular or linear extent of electrodes on one element is changed relative to the extent of electrodes on the opposing element, the impedance relationship between the elements is altered, necessitating the adjustment of related external circuitry to optimize the device's performance.